The present invention relates to closed circuit hydrostatic drives for propelling mobile working vehicles. More particularly, this invention relates to a loop flushing circuit for closed circuit hydrostatic transmissions.
In closed circuit hydrostatic transmissions that include at least one variable displacement pump, and at least one fixed or variable displacement motor, the power transmitting medium (typically oil) is kept contained within a closed loop circuit with the exception of leakage from the variable displacement pump(s) and the motor(s). A fixed displacement charge pump is generally used with the closed loop circuit to replace occurring leakage by pumping oil out of a separate oil reservoir and into the closed loop hydraulic circuit. The oil kept in the reservoir is normally of lower temperature than the oil removed from the closed loop circuit. This is because the oil in the reservoir generally has been cooled by means of an oil cooler also included in the overall system. The oil in the closed loop circuit should not exceed a temperature harmful for the functionality of the oil and the components used in closed loop circuit.
At certain operating conditions, however, especially when running hydrostatic transmissions at low operating loads (system pressure) concurrently with high pump and/or motor speed, additional means are necessary to maintain a desired operating temperature of the oil in the closed loop circuit. A high amount of heat can be generated within the closed circuit due to low leakage in the pump and motor at low operating loads, resulting in a small amount of oil being exchanged between the closed loop circuit and the reservoir. Also heat generation due to line losses within the closed circuit tends to be high because of the high speed condition. Conventional loop flushing valves are common means to allow for additional hot oil to be removed from the closed loop circuit, which results in a greater volume of oil being exchanged with the reservoir. This oil is replaced by means of a charge pump in the same manner as the oil removed from the closed loop circuit due to leakage in the pump(s) and motor(s). Secondly, the continuous exchange of oil from the closed loop circuit allows for the removal of small contamination particles from the closed loop circuit, generated during normal operation, by means of an oil filter also included in the overall system. Conventionally, a loop flushing valve is installed in either the pump(s) or motors)(s), and includes a spring-centered loop flushing spool shuttle valve and a loop flushing relief valve. The shuttle spool valve connects the loop flushing relief valve with that side of the closed loop circuit that operates at the lower of the two pressure levels of the closed loop circuit. Therefore, only oil from the low-pressure side can be drained through the loop flushing relief valve out of the closed loop circuit.
For many vehicles equipped with a closed loop hydraulic propulsion circuit, especially mobile working vehicles, operating the vehicles at low vehicle speed (creeping speed) in both driving directions while repeatedly changing driving direction from forward to reverse, and vice versa, is a necessary requirement when maneuvering around certain obstacles, or driving towards other vehicles or objects involved in the working process. In order to maintain predictable and repeatable vehicle performance under low vehicle speed operating conditions, it is required that the transmission of propel energy occurs steadily and without any sudden changes from the prime mover to the wheels or tracks of the vehicle. Having a loop flushing valve according to the prior art included in the propulsion system of said vehicles, however, can cause sudden changes of the transmission of energy resulting in unpredictable and nonrepeatable, and therefore uncontrollable vehicle movements. When driving a low vehicle speed and changing driving direction between forward and reverse or vice versa, the side of higher load (system pressure) in the closed loop propel circuit is changing from one side to the other and vice versa. Conventional loop flushing valves, the loop flushing relief valve are intended to be connected, via the loop flushing shuttle spool valve, to that side of the closed loop circuit that operates at the lower system pressure. This means the loop flushing shuttle spool has to shift every time the vehicle direction changes. Due to the dynamics of the loop flushing shuttle spool valve (springxe2x80x94massxe2x80x94damperxe2x80x94system), the shift of the shuttle spool is delayed, and does not occur instantaneously when the vehicle driving direction is changed. Therefore, for a short moment, the side of the closed loop circuit propel system with the higher operating pressure is connected to the loop flushing relief valve using some of the pressurized oil on the high pressure side of the closed loop circuit, intended for the transmission of hydraulic energy into mechanical energy within the hydraulic motor(s), to be flushed out of the closed loop circuit prior to being used in the intended way. When the loop flushing shuttle spool eventually shifts to the required position, connecting the low pressure side of the closed loop circuit to the loop flushing relief valve, the flushing flow out of the high pressure side of the loop is interrupted, and all of the pressurized oil on the high pressure side of the closed loop circuit is again transmitting hydraulic energy into mechanical energy within the hydraulic motor(s) as intended. This change, however, causes a sudden step increase in propel energy transmitted from the engine to the wheels of the vehicle, making its driving performance unpredictable and non-repeatable.
Beyond operating vehicles back and forth at low speed, vehicles very often are used predominantly for driving in only one direction, normally the forward direction. Others are equally used for driving forward and reverse, however, are only operated at low vehicle speed when going in reverse. Therefore, the closed loop hydraulic circuit cannot overheat while driving in reverse even when operating without any loop flushing valve.
From the foregoing it can be seen that there is a need for an improved loop flushing circuit. Therefore, a primary objective of the present invention is the provision of an improved loop flushing circuit.
Another objective of this invention is the provision of a loop flushing circuit that performs the usual loop flushing functions or requirements of cleaning and cooling without negatively interfering with the controllability requirements of hydrostatic propel systems.
Another objective of this invention is the provision of a loop flushing circuit that includes an electronically actuated loop flushing valve which can be controlled by a microprocessor and thereby adjusted through software or parameter alterations to optimize system operating temperatures for maximum performance and durability.
Another objective of this invention is the provision of a loop flushing device that has few components.
Another objective of this invention is the provision of a loop flushing circuit that is durable, reliable and economical to produce.
Another objective of this invention is the provision of a loop flushing circuit wherein loop flushing is adjustable depending on certain operating conditions such as control joystick position, pump displacement, motor speed, or pump output flow direction.
Another objective of this invention is the provision of a loop flushing circuit wherein loop flushing is defeated around neutral (where pump displacement is near zero).
Another objective of this invention is the provision of a loop flushing circuit that utilizes an electrical or electro-hydraulic two-position solenoid valve for selectively bleeding fluid from the high pressure system line to a case drain.
The present invention relates to a loop flushing circuit for a closed loop hydrostatic transmission. The loop flushing circuit includes a variable displacement hydraulic pump fluidly connected to a hydraulic motor by first and second system pressure lines, a case drain line associated with at least one of the motor and pump, and a two-port loop flushing valve fluidly connected to only one side of the loop.
The loop flushing valve has an open position in which loop oil is bled to the case drain line and a closed position wherein the valve blocks such flow.
The two-port valve can be solenoid-operated and controlled by a microcontroller that also controls the displacement of the pump and the direction of fluid flow in the closed loop circuit. The microcontroller electrically connects to the solenoid of the loop flushing valve and generates a signal to open the valve whenever a predetermined system condition or input value is achieved. The predetermined system operating condition or input value can include, but is not limited to, pump displacement, motor speed, control or joystick position. The microcontroller can be programmed so as to adjust or select the point at which the loop flushing valve is energized so that flushing occurs. Loop flushing can thus be defeated around the neutral condition of the transmission.